Identification and characterization of a recombinant cognate hemoglobin reductase from Synechocystis sp. PCC 6803

The globins of cyanobacteria and green algae: An update

The globin superfamily of proteins is ancient and diverse. Regular assessments based on the increasing number of available genome sequences have elaborated on a complex evolutionary history. In this review, we present a summary of a decade of advances in characterising the globins of cyanobacteria and green algae. The focus is on haem-containing globins with an emphasis on recent experimental developments, which reinforce links to nitrogen metabolism and nitrosative stress response in addition to dioxygen management. Mention is made of globins that do not bind haem to provide an encompassing view of the superfamily and perspective on the field. It is reiterated that an effort toward phenotypical and in-vivo characterisation is needed to elucidate the many roles that these versatile proteins fulfil in oxygenic photosynthetic microbes. It is also proposed that globins from oxygenic organisms are promising proteins for applications in the biotechnology arena.

A novel single sensor hemoglobin domain from the thermophilic cyanobacteria Thermosynechococcus elongatus BP-1 exhibits higher pH but lower thermal stability compared to globins from mesophilic organisms

Thermosynechococcus elongatus-BP1 belongs to the class of photoautotrophic cyanobacterial organisms. The presence of chlorophyll a, carotenoids, and phycocyanobilin are the characteristics that categorize T. elongatus as a photosynthetic organism. Here, we report the structural and spectroscopic characteristics of novel hemoglobin (Hb) Synel Hb from T.elongatus, synonymous with Thermosynechococcus vestitus BP-1. The X-ray crystal structure (2.15 Å) of Synel Hb suggests the presence of a globin domain with a pre-A helix similar to the sensor domain (S) family of Hbs. The rich hydrophobic core accommodates heme in a penta-coordinated state and readily binds an extraneous ligand(imidazole). The absorption and circular dichroic spectral analysis of Synel Hb reiteratedthat the heme is in Fe^III+ state with a predominantly α-helical structure similar to myoglobin. Synel Hb displays higher resistance to structural perturbations induced via external stresses like pH and guanidium hydrochloride, which is comparable to Synechocystis Hb. However, Synel Hb exhibited lower thermal stability compared to mesophilic hemoglobins. Overall, the data is suggestive of the structural sturdiness of Synel Hb, which probably corroborates its origin in extreme thermophilic conditions. The stable globin provides scope for further investigation and may lead to new insights with scope for engineering stability in hemoglobin-based oxygen carriers.

Characterization and Molecular Mechanism of a Novel Cytochrome b5 Reductase with NAD(P)H Specificity from Mortierella alpina

The electron-transfer capabilities of cytochrome b₅ reductase (Cyt b₅R) and NADPH supply have been shown to be critical factors in microbial fatty acid synthesis. Unfortunately, Cyt b₅R substrate specificity is limited to the coenzyme NADH. In this study, we discovered that a novel Cyt b₅R from Mortierella alpina (MaCytb5RII) displays affinity for NADPH and NADH. The enzymatic characteristics of high-purity MaCytb5RII were determined with the K_m,NADPH and K_m,NADH being 0.42 and 0.07 mM, respectively. MaCytb5RII shows high specific activity at 4 °C and pH 9.0. We anchored the residues that interacted with the coenzymes using the homology models of MaCytb5Rs docking NAD(P)H and FAD. The enzyme activity analysis of the purified mutants MaCytb5RII_[S230N], MaCytb5RII_[Y242F], and MaCytb5RII_[S272A] revealed that Ser230 is essential for MaCytb5RII to have dual NAD(P)H dependence, whereas Tyr242 influences MaCytb5RII's NADPH affinity and Ala272 greatly decreases MaCytb5RII's NADH affinity.

Nitric oxide represses photosystem II and NDH-1 in the cyanobacterium Synechocystis sp. PCC 6803

Photosynthetic electron transfer comprises a series of light-induced redox reactions catalysed by multiprotein machinery in the thylakoid. These protein complexes possess cofactors susceptible to redox modifications by reactive small molecules. The gaseous radical nitric oxide (NO), a key signalling molecule in green algae and plants, has earlier been shown to bind to Photosystem (PS) II and obstruct electron transfer in plants. The effects of NO on cyanobacterial bioenergetics however, have long remained obscure. In this study, we exposed the model cyanobacterium Synechocystis sp. PCC 6803 to NO under anoxic conditions and followed changes in whole-cell fluorescence and oxidoreduction of P700 in vivo. Our results demonstrate that NO blocks photosynthetic electron transfer in cells by repressing PSII, PSI, and likely the NDH dehydrogenase-like complex 1 (NDH-1). We propose that iron‑sulfur clusters of NDH-1 complex may be affected by NO to such an extent that ferredoxin-derived electron injection to the plastoquinone pool, and thus cyclic electron transfer, may be inhibited. These findings reveal the profound effects of NO on Synechocystis cells and demonstrate the importance of controlled NO homeostasis in cyanobacteria.

Expressed Soybean Leghemoglobin: Effect on Escherichia coli at Oxidative and Nitrosative Stress

Leghemoglobin (Lb) is an oxygen-binding plant hemoglobin of legume nodules, which participates in the symbiotic nitrogen fixation process. Another way to obtain Lb is its expression in bacteria, yeasts, or other organisms. This is promising for both obtaining Lb in the necessary quantity and scrutinizing it in model systems, e.g., its interaction with reactive oxygen (ROS) and nitrogen (RNS) species. The main goal of the work was to study how Lb expression affected the ability of Escherichia coli cells to tolerate oxidative and nitrosative stress. The bacterium E. coli with the embedded gene of soybean leghemoglobin a contains this protein in an active oxygenated state. The interaction of the expressed Lb with oxidative and nitrosative stress inducers (nitrosoglutathione, tert-butyl hydroperoxide, and benzylviologen) was studied by enzymatic methods and spectrophotometry. Lb formed NO complexes with heme-nitrosylLb or nonheme iron-dinitrosyl iron complexes (DNICs). The formation of Lb-bound DNICs was also detected by low-temperature electron paramagnetic resonance spectroscopy. Lb displayed peroxidase activity and catalyzed the reduction of organic peroxides. Despite this, E. coli-synthesized Lb were more sensitive to stress inducers. This might be due to the energy demand required by the Lb synthesis, as an alien protein consumes bacterial resources and thereby decreases adaptive potential of E. coli.

Multiple putative methemoglobin reductases in C. reinhardtii may support enzymatic functions for its multiple hemoglobins

The ubiquitous nature of hemoglobins, their presence in multiple forms and low cellular expression in organisms suggests alternative physiological functions of hemoglobins in addition to oxygen transport and storage. Previous research has proposed enzymatic function of hemoglobins such as nitric oxide dioxygenase, nitrite reductase and hydroxylamine reductase. In all these enzymatic functions, active ferrous form of hemoglobin is converted to ferric form and reconversion of ferric to ferrous through reduction partners is under active investigation. The model alga C. reinhardtii contains multiple globins and is thus expected to have multiple putative methemoglobin reductases to augment the physiological functions of the novel hemoglobins. In this regard, three putative methemoglobin reductases and three algal hemoglobins were characterized. Our results signify that the identified putative methemoglobin reductases can reduce algal methemoglobins in a nonspecific manner under in vitro conditions. Enzyme kinetics of two putative methemoglobin reductases with methemoglobins as substrates and in silico analysis support interaction between the hemoglobins and the two reduction partners as also observed in vitro. Our investigation on algal methemoglobin reductases underpins the valuable chemistry of nitric oxide with the newly discovered hemoglobins to ensure their physiological relevance, with multiple hemoglobins probably necessitating the presence of multiple reductases.